1. Field of the Invention
The present invention relates to a lead acid battery, and more particularly to a grid constituting electrodes of the lead acid battery.
2. Description of the Related Art
Because of, e.g., excellent stability in charging and discharging, lead acid batteries have been long used from the first commercialization, and a large number of the lead acid batteries are used. As compared with other system of storage batteries, however, the lead acid batteries have two main disadvantages that the energy density per weight is low and hence the weight of the battery itself has to be large. For this reason, although demand for lead acid batteries has increased as one type of portable power supplies in the recent trend toward cordless devices, there is a desire for lead acid batteries having more lighter weight, higher power and longer life than other battery systems. Conventional sealed lead acid batteries generally have the energy density per weight of about 34-35 WH/kg. However, a kind of lead acid battery having higher energy density of about 40-45 WH/kg is required for applications to an electric vehicle (EV), portable power supplies, etc.
In general lead acid batteries, an electrode plate is formed by coating lead oxide as a positive active material or lead as a negative active material on a grid made of lead or a lead alloy. The positive and negative electrode plates thus formed are positioned to face each other through a separator formed of glass fibers as a primary component, thereby fabricating a group of electrodes. The lead acid battery is then constructed by placing the group of electrodes in a container made of synthetic resin that is highly resistant against acid and shocks, and pouring dilute sulfuric acid as an electrolyte in the container.
Conventional grids for lead acid batteries have been manufactured by the casting method, the expanding method, the pressing method, etc. In any of these methods, other materials than lead or lead alloys have not been used as grid materials.
Of parts making up a lead acid battery, the part which can most effectively contribute to reducing the total weight of the lead acid battery without reducing the functions specific to that part is a grid. The grid of the lead acid battery is required to have three types of functions; (1) electrifying functions in charging and electricity collecting functions in discharging as electric conductors of the active materials in the processes of charging and discharging, (2) supporting functions of the grid itself and the active material, and (3) function of resisting corrosion ascribed to oxidizing and deoxidizing reactions occurred within the battery.
However, it has been very difficult for materials and structure of the grid to satisfactorily achieve the above functions of resisting vigorous oxidizing and deoxidizing reactions occurred within the battery, of enduring expansion and contraction of the electrode plates, and so on.